1. Field of the Invention
The present invention relates, in general, to monolithic devices for integrated circuits and, more particularly, to a monolithic high impedance device with programmable poles for use in passive low pass filters.
2. Relevant Background
Integrated circuits comprise a number of passive and active devices formed monolithically on a single integrated chip. Usually if more devices can be formed on a single chip the overall system cost for the electronic circuit using the chip is reduced. While great strides have been made in reducing the size of active devices such as transistors, passive devices such as resistors, have been somewhat more difficult to shrink. In particular, resistors are difficult to fabricate because high resistivity materials tend to be non-uniform leading to unpredictable resistor values.
Large value resistors are formed by long serpentine strings of resistive material. For resistance values larger than 10 megaohms, most manufacturers rely on off-chip, discrete resistors. The use of discrete components complicates circuit design, adds parasitics due to the interconnections between the integrated circuit and the discrete components, increases cost of the overall system and reduces reliability because of the additional connections. Moreover, valuable input-output leads to access the integrated circuit are dedicated for connections to the external passive components, limiting functionality of the integrated circuit and increasing cost of the packaging.
Passive low pass filters are widely used in voltage reference systems (e.g., power supplies, DC-DC converters, AC-DC converters, and the like). To effectively remove low frequency noise such as 1/f noise, a pole located at lower than 10 Hertz (Hz) is required. A "pole" is a point in the complex frequency plane where the magnitude of a given network function goes to infinity. Such a low frequency pole is usually achieved by a low pass filter using a resistor-capacitor combination. At least one and in some designs both the resistor and capacitor are implemented by off-chip components. These low pass filters provide a large time constant determined by the product of the resistance and capacitance used in the filter. On-chip resistors are usually limited to values of less than 10 megaohms because of the large area taken up by large value resistors.
What is needed is a low cost volume efficient structure and method for monolithically integrating large value resistors onto integrated circuits.